According to the prior art, such an NFC antenna onboard a vehicle is situated in a door handle of the vehicle and is made of two parts: a detection antenna and a communication antenna. The detection antenna consists of copper wire windings wound around the communication antenna. The detection antenna is linked to a microcontroller, generally situated in proximity to the detection antenna, and integrated into the door handle of the vehicle. The variation of the voltage across the terminals of this detection antenna, measured by the microcontroller, allows the detection of the approach of a portable device. The detection antenna is generally of large size and larger than the communication antenna so as to obtain a larger detection range.
The communication antenna is composed of copper wire windings and can comprise in addition some components such as an adaptation capacitor. The communication antenna is linked to an NFC reader integrated into the microcontroller. This communication antenna communicates with an NFC antenna situated in the portable device, at the near-field communication frequency, i.e. at a frequency of 13.56 MHz. It is said that the communication antenna resonates at 13.56 MHz. The adjustment of the frequency of the communication antenna is carried out, for example, by virtue of the adaptation capacitor. In order to avoid any phenomenon of coupling between the detection antenna and the communication antenna, due inter alia to the large size of the detection antenna, the latter does not resonate at the frequency of 13.56 MHz. The detection antenna therefore detects the approach of any portable device and not only the approach of portable devices equipped with an NFC antenna communicating at 13.56 MHz. It is understood that this creates false detections, since the detection antenna detects any type of portable device and not solely the portable devices with which the communication antenna can communicate. This results in an attempt of communication between the communication antenna and the portable device which will fail and which consumes energy.
The main criterion of the performance of an NFC antenna, onboard a vehicle, is the communication distance between this antenna, which will be called the primary NFC antenna, and the NFC antenna situated in the portable device, which will be called the secondary NFC antenna. Indeed, the range of the near-field communication (that is to say at a frequency of 13.56 MHz) goes from 0 cm (the two objects communicating with one another are then in contact) up to a maximum of 10 cm. It is understood that there is an advantage in being able to communicate with the portable device at the maximum distance allowed by an NFC communication, there then being no need for the user to make the portable device approach the door handle, still less to place it in contact with the door handle of the vehicle, to establish the communication.
The NFC communication distance is directly related to the size of the communication antenna and to the power of this communication antenna, provided by the microcontroller. Now, the power provided by the microcontroller to the communication antenna impacts directly on the electrical consumption of the vehicle. And during the vehicle stop phases, this electrical consumption must be minimized, so as not to discharge the battery. Any power increase in order to increase the communication distance of the communication antenna has to be avoided.
The size of the communication antenna is also limited, since this communication antenna, the detection antenna and the associated electronics (microcontroller, NFC reader, etc.), that is to say the detection and near-field communication device, is generally integrated into a very restricted space of the door handle of the vehicle. The integration of the two parts of the primary NFC antenna, that is to say of the detection antenna and of the communication antenna, is often difficult because of the lack of room in the allocated space of a vehicle door handle.
Thus, it is not possible either to increase the size of the communication antenna or to amplify the power provided to this antenna, in order to raise the distance of communication with the secondary NFC antenna of the portable device.
It is known from the prior art, to place a ferrite beneath the primary NFC antenna, said ferrite being situated vehicle side. This ferrite reflects that part of the electromagnetic field of the communication antenna which is directed toward the vehicle. The consequence of the reflection of the electromagnetic field toward the exterior of the vehicle is to substantially increase the communication distance of the communication antenna. However, the addition of a ferrite is of a non-negligible cost and is not always achievable in the door handle's restricted space dedicated to the detection and near-field communication device.’